Inside the flow clamp-on three pairs of piezoelectric sensors function both as a source and a detector for ultrasound waves. The sensor pairs are oriented diagonally in the corners of the clamp-on.
Each sensor sends a wave to its opposite partner. The waves travel through the flowing liquid at a certain angle, such that they either travel downstream and are accelerated by the flow stream or they travel upstream and are decelerated by the flow stream. This leads to a transit time difference between the two signals that is proportional to the flow velocity, from which the flow rate can be calculated using the inner diameter of the flow pipe. To increase measurement accuracy, the three sensor pairs measure the transit time difference simultaneously.